1. Field of the Invention
This invention relates generally to spectroscopy. More particularly, it relates to spectroscopic filters.
2. Description of the Related Art
Raman spectroscopy involves measuring a spectrum which is Raman shifted relative to an excitation wavelength. The excitation wavelength may be narrowband light (usually laser light) at any frequency convenient to the spectroscopist. When this excitation light scatters from the analyte and subsequently is detected by the spectrometer, it is called the Rayleigh line. The signal spectrum consists of a number of Raman shifted peaks, each at a distinct light frequency relative to the Rayleigh line. The magnitude of the frequency shift is determined by the nature of the molecular bonds of the particular analyte generating the peak. In order to obtain usable data from Raman shifted peaks, it is important that the spectrometer provide an accurate measurement of the shift of each peak.
In a spectrometer, every wavelength of light is imaged to a unique location on the spectrometer's sensor. The distance between nearby wavelengths is proportional to the dispersion of the spectrometer's dispersive element. Examples of dispersive elements include grating(s), prism(s), or grism(s). The spectrometer may be calibrated using measurements of reference light from a reference light source detected at different parts of the detector. An example of a reference light source is a neon light source. By tuning the wavelength of the reference light source and measuring the shift in location of the peak on the detector, the wavelength of every peak in a signal spectrum may be calibrated. Wavelength (λ) is easily converted to frequency (f) through the equation λ=nc/f; where n is index of refraction and c is the speed of light. Once the spectrometer has been calibrated, the frequency difference and Raman shift between peaks may be determined.
Correlating the measurements of the Raman shifted peaks to the measurement of the Rayleigh line is difficult, however. The Rayleigh line is traditionally not part of the signal. The Rayleigh line is so intense its presence saturates the spectrometer's detector, thereby making it very difficult to measure the Raman signal of interest. Typically, the Rayleigh line is physically blocked and/or is filtered from reaching the detector with optical interference filters that reduce the Rayleigh line by 8-12 orders of magnitude. Additionally, all non-imaged stray Rayleigh light is also blocked and/or filtered from reaching the detector to eliminate background glow, halos, or optical ‘ghosts’ which would otherwise obscure the Raman signal of interest.
Because the Rayleigh line is not incident on the detector, its wavelength/frequency must be measured with any number of alternate methods. Accurate calibration between the measurement of the Rayleigh line and the measurements received from the detector is a source of error that limits the accuracy of the spectrometer. In addition, a spectrometer's output may change over time if the spectrometer is disturbed in any way. For example, if the detector array shifts inside the instrument, e.g., caused by the device being dropped, then the spectrum will shift and the calibration will no longer be accurate. This leads to degradation in spectrometer accuracy over the life of the device.